In one method of magnetic resonance imaging, there is a method known as MR (Magnetic Resonance) spectroscopy.
Protons placed in an external static magnetic field are subject to an effective magnetic field due to the magnetic screening effect originating from the surrounding electron clouds of the protons, in which the magnitude of the effective magnetic field is different from that of the external static magnetic field, where this magnitude of the effective magnetic field varies depending on the chemical environment in which the protons are being placed.
In the magnetic resonance spectroscopy, a value quantifying the difference of the resonance frequencies due to the chemical environment, known as the chemical shift, is measured. In magnetic resonance spectroscopy choosing the brain as the imaging target, for example, protons of N-Acetylaspartic acid, creatine, Choline or the like are depicted.
However, protons of the hydroxyl group (—OH), the amide group (—NH) and the amino group (—NH2), for example, are invisible to the magnetic resonance spectroscopy.